Superprotonic Solid Acids: Thermochemistry, Structure, and Conductivity

Citation

Ikeda, Ayako (2013) Superprotonic Solid Acids: Thermochemistry, Structure, and Conductivity. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/R1SG-VY83. https://resolver.caltech.edu/CaltechTHESIS:09142012-115522353

Abstract

In this work, in order to investigate the thermochemistry and property of the superprotonic solid acid compounds, the measurement methods were established for in situ observation, because superprotonic phases are neither stable at room temperature nor freezable to room temperature. A humidity-controlled TG, DSC and AC impedance measurement system, and high temperature stage for XRD were built for thermal analysis and characterization of the solid acid compounds.

The thermodynamic and kinetics of the dehydration and hydration of CsH₂PO₄ is investigated by TG, DSC, and XRD analysis. By making use of the enhanced kinetics afforded by SiO2, the phase boundary between CsH₂PO₄, CsPO₃, and dehydrated liquid was precisely determined. The stability of CsH₂PO₄ and the liquid dehydrate, CsH_2(1-x)PO_4-x(l), were confirmed by the complete reversal of dehydration to recover these phases in the appropriate temperature and water partial pressure ranges. Rehydration and conversion of CsPO₃(s) to CsH₂PO₄(s) occurs over a period of several hours, depending on temperature, water partial pressure, and morphology of the metaphosphate. High and small particles favor rapid dehydration, whereas the temperature dependence of the rehydration kinetics is nonmonotonic, reaching its fastest rate in the vicinity of the superprotonic transition.

Doping Rb and K into CDP was examined and the stable region of Cs_1-xRbxH₂PO₄ and Cs_1-xKxH₂PO₄ are determined by in situ XRD and DSC measurement. Then the effects of doping to the structure and conductivity are discussed. It was found that Rb has whole-range solubility for both cubic and monoclinic CDP. Ts increases and Td decrease with Rb content. K has 27% solubility for cubic CDP, Ts and Td decrease with K content. The eutectic temperature is 208 ± 2°C. The lattice size of Rb- or K- doped CDP depends on the averaged cation size. Conductivity linearly decreases by dopant concentration. The impact of K doping is deeper than that of Rb for the equivalent averaged cation size.

in situ XRD measurement was carried out using single-crystal CsH₂PO₄ in order to study the phase transformation mechanism of this compound. A plate-like single crystal with (100) orientation was prepared, and the phase transition was observed by heating and cooling with ramp rate 0.2 K/min. From the obtained XRD profile — the after-first-phase transition (monoclinic-->cubic) — the distribution of the domain orientation was estimated. It was found that (100) is the preferential orientation after phase transition, however, the amount of the domains with other orientation is not ignorable. Therefore, it is considered that the phase transformation in CsH₂PO₄ is not simple martensitic, but that some other event, such as recrystallization, happens during the transition process.

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